Field of the Invention
The invention relates to mass spectrometers with pulsed or continuously emitting lasers whose laser light can be used for ionization by laser desorption, for the fragmentation of ions by photodissociation (PD), for the initiation of ion reactions, and other purposes.
Description of the Related Art
The Related Art is explained below with reference to a special aspect. This is not to be understood as a limitation, however. Useful further developments and modifications of what is known from the Related Art can also be used beyond the comparatively narrow scope of this introduction, and will easily be evident to the expert skilled in the art in this field after reading the following disclosure.
An important desorbing type of ionization for biomolecules is ionization by matrix-assisted laser desorption (MALDI), which was developed by M. Karas and K. Hillenkamp among others almost thirty years ago. MALDI ablates and ionizes the analyte molecules, preferably biomolecules, which are in highly diluted form in a mixture with molecules of a matrix substance in samples on sample supports, by bombarding them with pulses of laser light, usually pulses of ultraviolet (UV) laser light. Nitrogen lasers were previously the main type of laser for this task. Nowadays, however, the lasers used are predominantly solid-state lasers because they have a far longer lifetime and higher pulse frequencies. The lasers usually used have neodymium-doped crystals and the photon energy is tripled by non-linear crystals; for example with a target wavelength of 355 nanometers, starting from 1064 nanometers.
The ions which are created in the plasma of each pulse of laser light are, in most cases, accelerated in specially designed MALDI time-of-flight mass spectrometers (MALDI-TOF-MS) with between 20 and 30 kilovolts and axially injected into a flight path. After passing through the flight path, they encounter an ion measuring system, which measures the mass-dependent arrival time of the ions and their quantity, and then records the digitized measurements in the form of a time-of-flight spectrum. In the past, nitrogen lasers were used with repetition rates of between 20 and 60 hertz for the laser light pulses. Solid-state lasers were used with repetition rates of up to 2,000 light pulses per second. Recently, the Applicant developed a MALDI-TOF mass spectrometer with light pulse frequencies and spectral acquisition frequencies of 10 kilohertz.
An important issue in ion detection is to prevent saturation effects. To this end, the number of ions generated per pulse of laser light is usually limited, for example to a maximum of only a few thousand per pulse. A few hundred to a few thousand individual spectra are therefore summed for a time-of-flight spectrum. The mass spectra can achieve mass resolutions of R=m/Δm=80,000 and more nowadays, where Δm is the width of the ion signal at half height.
Using suitable MALDI time-of-flight mass spectrometers, it is also possible to acquire fragment ion spectra which are generally based on a decomposition of the formed ions because of excess internal energy. These fragment ions can be post-accelerated and measured as fragment ion spectra. Unfortunately, the yield of fragment ions is low with this decomposition process. It is therefore of interest to be able to use other types of fragmentation also.
A conventional type of fragmentation is collision-induced dissociation (CID), sometimes also called collision-activated dissociation (CAD), in which accelerated ions are injected into a collision cell and fragmented there by means of collisions at pressures of a collision gas, such as molecular nitrogen, of 10−3 to 10−1 pascal. A special type of time-of-flight mass spectrometer is required for this, however. FIG. 1 shows a well-known time-of-flight mass spectrometer with orthogonal acceleration of the ions (“OTOF”) by a pulser. This time-of-flight mass spectrometer is equipped with a MALDI ion source and a collision cell on the ion path to the orthogonal pulser.
FIG. 1 shows a simplified schematic representation. The usual normal operating mode with temporary storage and possible collision-induced dissociation of the ions in the ion storage device (9) looks as follows: In an ion source with laser system (1), ions (6) are produced from the sample on the sample support plate (5) by the beam of pulsed laser light (2), which enters the source through a window (3), said ions then being pushed by a potential at the electrode (4) into a conventional high-frequency (RF) ion funnel (7). The ions then enter the RF quadrupole rod system (8), which can be operated both as a simple ion guide and as a mass filter to select a species of parent ion to be fragmented. The unselected or selected ions are then fed into the RF quadrupole ion storage device (9) and can be fragmented there by high-energy collisions according to their acceleration. The ion storage device (9) has a gastight casing and is charged with collision gas, such as nitrogen or argon, through the gas feeder (10) in order to focus the ions by means of collisions and to collect them in the axis.
At specified times, ions are extracted from the ion storage device (9) by the switchable extraction lens (11), which shapes the ions into a fine primary beam (12) and sends them to the ion pulser (13). The ion pulser (13) pulses out a section of the primary ion beam (12) orthogonally into the high-potential drift region, thus generating the new ion beam (14). The ion beam (14) is reflected in the reflector (15) using velocity focusing and measured in the detector (16). The mass spectrometer is evacuated by the connected pumps (17), (18) and (19).
A further type of fragmentation is photodissociation (PD). In this process, ions can be fragmented by photons, either by a single absorbed photon of sufficiently high energy (typically in the UV range) or by many photons which have to be absorbed at practically the same time (multiphoton dissociation, MPD). A separate PD laser is typically used to generate these photon beams for fragmentation, see for example Choi et al., J. Am. Soc. Mass Spectrom. 2006, 17, pp. 1643-1653; see FIG. 1 there.
Two laser systems are occasionally used in other types of mass spectrometer also, in order to both produce fragment ions. For example, Watson et al. (Anal. Chem. Vol. 59, No. 8, Apr. 15, 1987) describe the use of a pulsed CO2 laser for desorption and ionization, and a continuous CO2 laser for fragmentation in an ion cyclotron resonance mass spectrometer (ICR-MS); see FIG. 3 there.
The use of the laser beam of a pulsed UV laser by intermittent deflection with a mirror device, both for MALDI ionization and for the fragmentation of temporarily stored ions, has also been disclosed: see FIGS. 4(a) and 4(b) in U.S. Pat. No. 7,351,955 B2 (V. V. Kovtoun, 2005). The same beam of UV laser light with identical wavelength is used for both purposes.
The splitting of a UV pulsed laser beam to be used simultaneously for the ionization and fragmentation is furthermore known from the paper “Photodissociation Studies of Small Peptide Ions by Fourier Transform Mass Spectrometry” by G. S. Gorman and I. J. Amster (Organic Mass Spectrometry, Vol. 28, 437-444, 1993).
Furthermore, the ionization and fragmentation of the molecules of an H2 molecular beam by laser photons of the two wavelengths ω and 2ω (780 nanometers, 150 femtoseconds) from a Ti:SA CPA ultrashort pulsed laser with temporal offset of the beam pulse with frequency ω with respect to the beam pulse with 2ω, has been disclosed. The two beam pulses are, however, sent to the same point and combined there. (Y. L. Shao et al., Journal of Modern Optics, 43:5, 1063.1070; “Two-color strong-field ionization and dissociation of H2 using 780 and 390 nm femtosecond pulses”).
A similar arrangement, but using two pulsed lasers, one excimer and one dye laser, was used to investigate the dynamic behavior of the photodissociation of CH2BrCl (W. S. McGivern et al., J. Chem. Phys., Vol. 111, No. 13, 1999; “Photodissociation dynamics of CH2BrCl”).
In the field of mass spectrometry, applications with two laser systems are known for the ionization of molecules and the fragmentation of the resulting ions at different locations. Also known are applications using only one laser system to ionize molecules and fragment ions with photons of the same energy at different locations with the aid of beam splitting or moving mirror systems.